With Russia extending its air strikes in Syria to include the ancient city of Palmyra, which is being held by Islamic State, Turkish President Tayyip Erdogan said he was losing patience with Russian violations of his country’s air space.

Nato Secretary-General Jens Stoltenberg said the alliance had reports of a “substantial build-up of Russian forces in Syria”, including ground troops and ships in the eastern Mediterranean.

Referring to the air incursions over Turkey’s border with Syria, he said: “I will not speculate on the motives… but this does not look like an accident and we have seen two of them.”

‘Extremely dangerous’

The incidents, which Nato has described as “extremely dangerous” and “unacceptable”, raise the risk of a further escalation of the Syrian civil war, with Russian and US aircraft flying combat missions over the same country for the first time since World War Two.

Russia has said that an SU-30 warplane entered Turkish air space along the border with Syria “for a few seconds” on Saturday, a mistake caused by bad weather. Nato said a plane also entered Turkish air space on Sunday, an incident Russia says it is looking into.

The US and Russia are at odds over Moscow’s involvement in the Syrian conflict, witn Moscow saying it is attacking Islamic State and Washington accusing it of hitting rebel groups opposed to the Assad regime.

Russia has said it agrees in principle with US proposals on coordinating military flights in Syria to avoid accidents.

Civil war

President Vladimir Putin has said he will not send ground troops to Syria, where a civil war has led to the deaths of 250,000 people. But Nato believes there is now evidence of a Russian build-up.

The Russian air campaign, which began almost a week ago, has targeted Palmyra and the northern province of Aleppo, Syrian state television said today.

Islamic State forces captured Palmyra in May, since when it has destroyed several of the city’s Roman-era ruins.